A study of 347 patients with gastrointestinal diseases revealed elevation of CA125 in sera of 63% of patients with pancreatic carcinoma, 46% of patients with carcinoma of the biliary tract, 40% of patients with liver carcinoma and 11-37% of patients with other carcinomas. All of the patients with acute pancreatitis, chronic pancreatitis, cholelithiasis, and peptic ulcer had normal CA125 values, but 35% of patients with liver cirrhosis and 10% of patients with chronic active hepatitis had elevated values. Patients with disseminated carcinomas had significantly higher levels than patients with localized carcinomas. CA125 did not significantly correlate with CA19-9 or carcinoembryonic antigen in patients with pancreatic carcinoma. Ninety-seven percent of patients with pancreatic carcinoma were defined as being positive when both serum CA125 and CA19-9 were evaluated. These results indicate that CA125 is useful for differentiating pancreatic carcinoma from chronic pancreatitis, especially when supplemented with CA19-9.